1. Field of Invention
This invention relates to systems and methods for recording of phonocardiogram (PCG) signals at locations distal to the heart.
2. Description of Related Art
Sound signals recorded from the heart are called phonocardiogram (PCG) signals. A healthy heart generates two distinct sounds that are coincident with the closure of the heart valves. These two sounds are called the first heart sound (S1) and the second heart sound (S2). In particular, S2 is coincident with the closure of the aortic valve. The closure of the aortic valve also creates a distinct feature on the arterial blood pressure waveform called the dicrotic notch. The pulse transit time (PTT) is defined as the time required for an arterial blood pressure pulse to travel from the heart to a distal location to the heart. The PTT can be measured by determining the delay between the closure of the aortic valve and the arrival of the dicrotic notch at a distal measurement point. This measurement is possible since, by placing a phonocardiogram (PCG) sensor on the chest near the aortic valve, it can be assumed that the delay between the closure of the valve and the measurement of the S2 sound on the chest is negligible. The arterial blood pressure pulse wave velocity (PWV) is computed by dividing the arterial path length between the heart and the measurement point by the PTT. Because the PWV is dependent on the stiffness of the arterial walls, it can be a useful parameter in determining the health of the cardiovascular system (e.g. arteriosclerosis).
Conventional methods and devices measure PWV using a PCG sensor in a standard location on the chest and a blood pressure waveform measurement at points distal to the heart. When PTT/PWV is to be measured using a PCG sensor at a location distal to the heart, the true PTT/PWV cannot be calculated unless one accounts for the propagation delay, hereinafter referred to as S2D, where S2D is the measured delay between the valve closure and the arrival of second heart sound (S2) at the distal location. The accuracy of the PTT/PWV measurement is directly proportional to the accuracy with which S2D is known.